Hydraulic levelling apparatus for automotive vehicles and shut-off valve for the same

ABSTRACT

An automotive shut-off valve arranged between the control valve for hydraulic suspension elements of a vehicle and an enginedriven hydraulic pump prevents leakage of oil from the suspension elements to the sump when the engine is not operated although oil may leak through the valve controlling oil flow toward and away from the suspension elements in response to the relative vertical position of the sprung and unsprung vehicle masses, as is necessary for sensitive response of the control valve to changes in vehicle level. The shut-off valve has two spring-biased valve members of which one opens a passage between the hydraulic pump and the control valve when the pump is operated, and the other one is moved by a mechanical connection to an additional valve member to open a passage between the sump and the control valve. The additional valve member closes a throttled passage connecting the pump discharge to the sump and moves the other valve member against its biasing spring as the pressure of pumped liquid builds up in the shut-off valve, and the additional valve member is shifted back to its starting position by the other springbiased valve member when the pump is shut down.

United States Patent 19 1' SpichaIa HYDRAULIC LEVELLING APPARATUS FORAUTOMOTIVE VEHICLES AND SHUT-OFF VALVE FOR THE SAME [75] Inventor: PaulSpichala, Schweinfurt am Main, Germany [73] Assignee: Fichtel & SachsAG, Schweinfurt am Main, Germany [22] Filed: Mar. 7, 1972 [21] Appl.No.: 232,570

[30] Foreign Application Priority Data Mar. 20, 1971 Germany P 21 13596.6

[52] US. Cl 280/124 F, 280/6 R [51] B60g 13/06 [58] Field of Search280/124 F, 124 R, 280/6 R, 6.1; 137/596, 596.11, 596.12

[56] References Cited UNITED STATES PATENTS 2,475,298 7/1949 Sloane137/596 3,492,013 1/1970 Allen 280/6 R 3,032,350 5/1962 Ruhl et a1280/124 F FOREIGN PATENTS OR APPLICATIONS 1,286,414 l/1969 Germany280/124 1,159,282 12/1963 Germany 280/124 1,113,146 8/1961 Germany280/124 [451 Jan. 1,1974

Primary Examiner-Philip Goodman Assistant Examiner1ohn A. CarrollAttorney-Kurt Kelman et a1.

[57] ABSTRACT sponse of the control valve to changes in vehicle level.

The shut-off valve has two spring-biased valve members of which oneopens a passage between the hydraulic pump and the control valve whenthe pump is operated, and the other one is moved by a mechanicalconnection to an additional valve member to open a passage between thesump and the control valve. The additional valve member closes athrottled passage connecting the pump discharge to the sump and movesthe other valve member against its biasing spring as the pressure ofpumped liquid builds up in the shut-off valve, and the additional valvemember is shifted back to its starting position by the otherspringbiased valve member when the pump is shut down.

3 Claims, 6 Drawing Figures PATENTEDJAH 1 1974 SHEET 3 OF 4 Fig.6

HYDRAULIC LEVELLING APPARATUS FOR AUTOMOTIVE VEHICLES AND SHUT-OFF VALVEFOR THE SAME This invention relates to levelling apparatus forautomotive vehicles operated at least in part by means of hydraulicfluid, to a hydraulic system for such apparatus, and to 'a shut-offvalve arrangement in the-system.

It is known to provide automotive vehicles with hydraulic orhydropneumatic suspension elements and with a hydraulic system thatsupplies fluid to the suspension elements and drains fluid from theelements in response to changes in the vertical spacing of sprung andunsprung masses of the vehicle which affect the effective length of thesuspension elements.

The direction of flow between a source of hydraulic fluid, such as apressure pump and a sump or storage container for the hydraulic fluidand the suspension elements, is automatically controlled by a valve setby a sensing device responsive to the relative vertical position of thesprung and unsprung masses of the vehicle. In order not to affect theride of the vehicle, the sensing device can provide only minimal forcesfor setting the control valve, and it is impractical to build aflowreversing control valve capable of being operated by minimal forces,and yet tightly sealing the several associated conduits from each other.

If such conduits directly connect the control valve to the pump and tothe sump, and if the pump is driven by the vehicle engine, as iscustomary, hydraulic fluid leaks from the suspension elements to thesump when the engine is stopped, and it is necessary to run the enginefor a short time after starting to make up for the leakage loss and torestore the necessary effective length of the suspension elements. Asimilar problem arises when the unsprung masses are relieved of theweight of the sprung masses.

A primary object of the invention is the provision of a hydraulic systemfor levelling apparatus of the type described which maintains theeffective length of the suspension elements during shut-down of theengine.

A more specific object is the provision of a shut-off valve suitable forbeing arranged between the hydraulic pressure pump and the control valveof the levelling arrangement which automatically prevents discharge ofhydraulic fluid toward the sump from the suspension elements, such ashydraulic'or hydropneumatic shock absorbers and springs when the pumppressure subsides.

The shut-off valve arrangement according to the invention has four portswhich may connect the valve to respective conduits for flow of fluidbetween the valve and the conduits. They are a pressure port, a feedport, a return port, and a drain port. A first valve is operativelyinterposed between the pressure and drain ports and responds to flow offluid from the pressure port toward the drain port by moving toward aclosing position in which it seals the two ports from each other, andthereafter responds to higher pressure at the pressure port than at thedrain port for maintaining its closing position.

A second valve is operatively interposed between the return port and thedrain port and spring-biased toward a closing position in which it sealsthe return port from the drain port. A mechanical linkage connects thetwo valves in such a manner as to move either valve away from itsclosing position in response to movement of the other valve into therespective closing position. A

third valve is interposed between the pressure and feed ports and isspring-biased toward a closing position in which it seals the feed andpressure ports from each other, but the biasing spring yields when thefluid pressure at the pressure port exceeds the fluid pressure at thefeed port by a predetermined amount.

When this shut-off valve arrangement is assembled in a hydraulic systemwith a pressure pump and a container adapted to hold hydraulic fluid, asuction conduit connecting the pump intake with the container, apressure conduit connecting the pressure discharge of the pump with thepressure port of the valve arrangement, and a drain conduit connectingthe drain port to the container, the feed and return ports areautomatically sealed from the pump and the container when the pump stopsoperating.

The hydraulic system may therefore be incorporated to advantage in alevelling apparatus for an automotive vehicle which has an engine fordriving the aforementioned pump, ahydraulic suspension elementinterposed between the sprung and unsprung masses of the vehicle,resilient means biasing the suspension element in a direction to movethe masses vertically away from each other, a feed conduit, a returnconduit, and a control valve interposed between the suspension elementand the feed and return conduits which connects the element to theseconduits respectively in response to changes in the vertical spacing ofthe sprung and unsprung masses. The feed conduit is connected to thefeed port in the shut-off valve arrangement of the hydraulic system, andthe return conduit to the return port. In the assembled apparatus, abody of hydraulic fluid substantially fills the suspension element, thefeed conduit, the return conduit, and the control valve under alloperative conditions.

Other features, additional objects, and many of the attendant advantagesof this invention will readily be appreciated as the same becomes betterunderstood by reference to the following detailed description of apreferred embodiment when considered in connection with the appendeddrawing in which:

FIG. 1 diagrammatically illustrates a levelling arrangement for anautomotive-vehicle;

FIG. 2 shows a shut-off valve in the arrangement of FIG. 1 inelevational section;

FIG. 3 illustrates the valve of FIG. 2 in plan view;

FIG. 4 shows the valve of FIG. 2 in a different operating position; v 7

FIG. 5 shows the level control valve of the arrangement of FIG. 1inelevational section; and

FIG. 6 shows a shock absorber and an associated pneumatic spring of thearrangement of FIG. 1 in section.

Referring now to the drawing in detail, and initially to FIG. 1, thereis shown a hydraulic pressure pump 20 which is driven by the vehicleengine M whenever the engine operates. The pump 20 draws hydraulicfluid, hereinafter referred to as oil for the sake of bravity, through asuction conduit and its intake from a storage container or sump 21, anddischarges it under pressure toward a shut-off valve 22 and a levelcontrol valve 23 through a pressure line 24 connecting the pump to theshut-off valve. When the shut-off valve does not forward the oil to thecontrol valve 23, it returns the oil to the sump 21 through a drain line25. Oil flows between the valves 22, 23 through a feed line 26 and areturn line 27. Two connecting lines 29 communicate each with apneuamticspring 30 and -a shock absorber 31. Oil leaking from each shock absorber31 is directed to the shut-off valve 22 through a hose 28.

One of the two identical shock absorbers 31 and its associated pneumaticspring 30 are seen in more detail in FIG. 6. The spring 30 is acontainer divided into two chambers 64, 65 by a flexible diaphragm 30.The chamber 64 is filled with oil and communicates with the connectingline 29, and the chamber 65 is sealed and filled with compressed air ornitrogen.

The shock absorber 31 has a cylinder 60 and a coaxial piston rod 61respectively fastened to the sprung and unsprung masses of the vehiclein a conventional manner, not illustrated. The inner end of the pistonrod 61 is fastened to a piston 62, and the compartment 63 of thecylinder cavity remote from the piston rod 61 communicates directly withthe connecting line 29. The compartment 63 is connected with the annularcylinder compartment about the piston rod 61 through axial throttlingpassages 66 in the piston 62.

The annular cylinder cover 69 is sealed to the piston rod 61 by twoaxially spaced sealing rings 70, 71 which axially bound an annularchamber 72 radially separating the piston rod 61 from the cover 59. Thechamber 72 communicates with the hose 28.

FIG. shows the level control valve 23 which directs the flow of oilbetween the two shock absorbers 31 and the shut-off valve 22. The valve23 has a slide 68 axially movable in a tubular housing 68. The twoconnecting lines 29 enter the valve housing 68' on a common axial levelspaced from the orifice of the return line 27. The

orifice of the feed line 26 is axially intermediate the orifices of thelines 27 and 29. The valve slide 28 is moved axially in the sleeve orhousing 68' by a linkage 67 connected to a device which senses thedistance between the sprung and unsprung masses of the vehicle andshifts the valve slide 68 in a direction to restore a normal distance,as is conventional in itself and not shown in detail.

In the illustrated position of the control valve 23, oil is sealed inthe shock absorbers 31 except for minor leakage past the slide 68, whichmust move freely in the housing68', and oil reaching the valve housing68' from the valve 23 through the feed line 26 is directly returned tothe valve 22 through the return line 27. When the distance between thesprung and unsprung masses becomes too small, the valve slide 68 isshifted toward the right, as viewed in FIG. 5, until it seals theorifice of the return line 27 and connects the feed line 26 to theconnecting lines 29 and the shock absorbers 31 to expand the latter.When the distance is excessive, the slide 68 moves far toward the left,until all lines26, 27, 29 communicate with each othenand oil from theshock absorbers 31 can flow out through the return line 27 together withthe oil supplied by the feed line 26.

At the core of this invention is the shut-off valve 22 whose body hastheshape of a square metal plate 22', as is evident from jointconsideration of FIGS. 2 and 3, and which is formed with variouslyconnected straight bores in a common median plane parallel to the squarefaces and having respective orifices in the narrow edge faces.

A first bore 32 vertically passing through the valve plate 22 begins ata threaded port 24 in the bottom face which normally receives thepressure line 24 coming from the pump 20. The analogous port of the bore32 in the top face of the valve plate is sealed by a threaded plug 18.The bore 32 is of stepped cylindrical shape, and its crosssectiondecreases from both ports inward of the valve plate 22'. The plug18 provides an abutment for a helical compression spring 11 which biasesa. conical valve member 10 toward a seat on a shoulder in the first bore32. The portion of the first bore which receives the spring 11communicates with a threaded feed port 26' in the left side face of thevalve plate 22', as viewed in FIG. 2. The feed line 26 is connected tothe port 26' in the assembled condition of the valve 22.

.T he two ends of a second, horizontal through-bore 9 of stepped,cylindrical configuration are sealed by respective threaded plugs 18',18". The wide portion 5 of the second bore 9 adjacent the plug 18'intersects the first bore 32 between the valve 10 and the port 24'. Aspherical valve member 6 is received with clearance in the wide boreportion 5 and prevented from blocking the intersection of the first andsecond bores by a narrow abutment pin 35 on the plug 18. In thecondition of the valve 22 seen in FIG- 2, the valve member 6 is held offits seat at the inner end of the wide bore portion 5 by a slotted pin 7guided in the narrowest portion of the horizontal second bore 9 andfixedly fastened to a conical valve member 8, which is shown in FIG. 2,to seal a wide bore portion 16 adjacent the plug 18" under the pressureof a helical compression spring 12 interposed between the plug 18" andthe valve member 8.

An end of the drain line 25 is normally inserted in a threaded drainport 25 in the narrow bottom face of the valve plate 22' andcommunicates with the horizontal second bore 9 in the portion of thelatter between the seats of the valve members 6 and 8 through a verticalbore 44 so that liquid may flow between the pressure port 24' and thedrain port 25 through the slot in the pin 7.

Another horizontal bore 45 extends from an orifice in the right sidewall of the valve plate, closed by a threaded plug 19, to the first,vertical bore 32 intermediate the pressure port 24' and the wide portion5 of the second,'horizontal bore, and is normally closed by a sphericalpressure-relief valve 15 under the force of a helical compression spring17 The bore 45 intersects the bore 44, thereby permitting oil flowbetween the ports 24' and 25 when the pressure in the port 24 becomesexcessive and opens the valve 15.

A returnport 27 for the return line 27 in the top face of the valveplate 22 is connected with the wide portion 16 of the second, horizontalbore 9 and with two nipples 13on the top face and right side facerespectively of the valve plate. The nipples 13 are equipped with checkvalves 14 preventing outward flow of oil from the valve 22 through thenipples 13 which normally attach the hoses 28 to the shut-off valve 22.

Mounting holes 53 connect the square faces of the valve plate 22' andnormally receive bolts which attach the valve 22 to the vehicle.

The apparatus described above operates as follows:

When the engine is started while the shut-off valve 22 is in thecondition seen in FIG. 2, in which the valve 6 is open and the valves 8and 10 are closed, oil initially can flow through the narrow slot in thepin 7 from the pressure port 24' to the drain port 25'. Because of thethrottling effect of the slot and of the valve member 6, the pressurerises in the wide portion 5 of the bore 9, and the valve member 6ultimately engages its seat and 8 toward the right, thereby interruptingdirect communication between the ports 24', 25' and opening a flow pathbetween the return port 27' and the drain port 25'.

As the oil pressure further rises in the bore portion 5, it canultimately overcome the restraining force of the valve spring 11 andlift the conical valve member from its seat to permit oil flow from thepressure port 24 to the feed port 26. Oil now flows from the pumpthrough the lines 24, 26 to the control valve 23, and can return to thesump 21 through the lines 27, 25. Friction in the lines maintains apressure differential sufficient to keep the valve member 6 seated andthe valve member 8 in the open position against the force of the spring12. This condition of the valve 22 is shown in FIG. 4.

If the control valve 23 is in the position shown in FIG. 5 correspondingto a normal distance between the sprung masses (body or frame) of thevehicle and the unsprung masses (axle and wheels) in which the pneumaticsprings 30 are adequately effective, the oil moves from the feed line 26directly to the return line 27 through the 'valve body 68.

If the distance is too small, as under an increased static of dynamicload, the oil flowing under pressure through the feed line 26 issupplied by the valve 23 to the shock absorbers 31 for expanding thesame until the normal spacing of body and axle is restored, and thevalve slide 68 is returned to the illustrated position. If the distanceis excessive, oil is returned from the shock absorbers 31 to the sump 21as described above to shorten the effective length of the shockabsorbers. No changes occur in the shut-off valve 22 during the threebasic modes of operation of the control valve 23.

When the engine M and the pump are stopped, the pressure at the port 24drops and cannot keep the valve 10 in the open position against itsspring 11. As the pressure in the wide bore portion 5 decays, the spring12 can return the valve members 8 and 6 to the respective positions seenin FIG. 2.

Because of the required free movement of the valve slide 68 in thehousing 68' of the control valve 23, oil now tends to flow from thepneumatic springs 30 and the shock absorbers 31 toward the valve 22through the lines 26, 27 as the oil leaks past the valve slide 68 underthe pressure supplied by the gas cushions in the chambers 65 and theweight of the vehicle body. In the absence of the shut-off valve 22, oilwould be discharged slowly from the springs 30 and shock absorbers 31into the sump 21. However, all communication between the shock absorbers31 and the sump 21 is interrupted by the seated valve members 8, 10, andthe higher fluid pressure in the shock absorbers and communicatingelements helps to keep the valve members 8, 10 in sealing engagementwith their seats.

During normal operation, the nipples 13 are connected to the drain portby the open check valves 14, and oil leaking past the sealing rings 70into the chambers 72 during oscillation of the pistons 62 is dischargedto the sump 21 through the hoses 28. When the engine is shut off, andthe pressure rises in the bore portion 16 behind the valve 8, the checkvalves 14 are closed, and oil under pressure cannot flow into thechambers 72. Significant amounts of oil do not leak through the ring 70when the piston rod 61 stands still, and such minute amounts of oil thatmay still enter the chamber 72 from the. cylinder are retained by thesealing ring 71.

The shut-off valve 22 thus maintains the normal operating level of thesprung vehicle masses when the engine is shut off. It is also effectivewhen the vehicle is raised by a service station lift which acts on theframe or body directly, so that the axles and wheels dependfreely fromthe frame, and the control valve slide 68 is shifted toward the leftfrom the position seen in FIG. 5. In the absence of the valve 22, thecompressed gas in the pneumatic springs 30 would drive the oil from thesprings into the sump 21 so that the vehicle, when lowered to its wheelsby the lift, would come to rest on much shortened shock absorbers 31.When the engine is then started, and the vehicle is driven from thelift, its bottom may hit a low projection on the ground before the shockabsorbers can be restored to their normal length. Partial emptying ofthe shock absorbers 31 in the raised vehicle, however, is impossiblewith the valve 22 in which the connection between the lines 26, 27 andthe sump 21 is interrupted shortly after the engine is shut off.

The pressure relief valve 15 prevents damage to the apparatus by oilunder excessive pressure in the event of malfunctioning, as isconventional in itself.

While the shut-off valve 22 has been described in its application to alevelling arrangement for an automotive vehicle, other uses in hydraulicequipment will readily suggest themselves. The use of the valve in ahydraulic press is specifically contemplated and has obvious advantages.

It should be understood, therefore, that the foregoing disclosurerelates only to a preferred embodiment, and that it is intended to coverall changes and modifications of the example of the invention hereinchosen for the purpose of the disclosure which do not constitutedepartures from the spirit and scope of the invention set forth in theappended claims.

The basic principle of this invention can with respect to FIGS. 1 and 2be regarded as follows:

Lines 24 and 26 together are a feed conduit; lines 27 and 25 togetherare a return conduit. In return conduit 27, 25 is provided a shut-offvalve 8. This shut-off valve 8 is controlled by a valve member 6 viaslotted pin 7 so that shut-off valve 8 is open when a certain pressureis exceeded at the pressure discharge of the pump 21 and is closed whenthe pressure at said pressure discharge is below said predeterminedvalue. The valve member 6 can therefore be regarded as(pressure-sensitive control means for the shut-off valve 8.

What is claimed is:

1. In a levelling apparatus for an automotive vehicle having an engine,a hydraulic suspension element interposed between the sprung and theunsprung masses of the vehicle, resilient means biasing said element ina direction to move said masses vertically away from each other, a feedconduit, a return conduit, and control valve means interposed betweensaid element and said feed and return conduits for connecting saidelement to said feed and return conduits respectively in response tochanges in the vertical spacing of said masses, the improvement whichcomprises:

a. a shut-off valve arrangement including 1. a plurality of port meansfor connecting the valve arrangement to respective conduits for flow offluid between the valve arrangement and said conduits, said port meansincluding pressure port means, feed port means, return port means, anddrain port means,

2. first valve means operatively interposed between said pressure portmeans and said drain port means and responsive to a flow of fluid fromsaid pressure port means toward said drain port means for moving towarda closing position in which said first valve means seals said pressureport means from said drain port means, and thereafter responsive tohigher pressure at said pressure port means than at said drain portmeans for maintaining the closing position thereof,

3. second valve means operatively interposed between said return portmeans and said drain port means,

4. yieldably resilient means biasing said second valve means toward aclosing position in which said second valve means seals said return portmeans from said drain port means,

5. motion transmitting means interposed between said first and secondvalve means for moving each of said valve means away from the closingposition thereof in response to movement of the other valve means intothe respective closing position,

6. third valve means interposed between said pressure port means andsaid feed port means; and 7. yieldably resilient means biasing saidthird valve means toward a closing position in which said third valvemeans seals said feed port means from said pressure port means, butadapted to yield when the fluid pressure at said pressure port meansexceeds the fluid pressure at said feed port means by a predeterminedamount;

b. a pressure pump having an intake and a pressure discharge; 0. acontainer adapted to hold hydraulic fluid;

d. a suction conduit connecting said intake to said container;

e. a pressure conduit connecting said pressure discharge to saidpressure port means;

f. a drain conduit connecting said drain port means to saidcontainer, 1. said engine being drivingly connected to said 2. said feedconduit being connected to said feed port means, and 3. said returnconduit being connected to said return port means; and g. a body ofhydraulic fluid substantially filling said element, said feed conduit,said return conduit, and said control valve means.

2. In an apparatus as set forth in claim I, wherein said element has twomembers movable relative to each other and jointly bounding a cavitysubstantially filled with fluid of said body, the improvement furthercomprising a chamber bounded by one of said members and located forcollecting fluid leaking from said cavity, a leakage conduitcommunicating with said chamber, and a check valve operativelyinterposed between said leakage conduit and said return port means forconnecting the leakage conduit to said return port means only when thefluid pressure in the return port means is not greater than in saidleakage conduit.

3. In an apparatus as set forth in claim 1, pressure relief valve meansinterposed between said pressure port means and said drain port meansfor directly connecting said last-mentioned port means, and yieldablyresilient means holding said relief valve means in a closed position andthereby preventing a direct connection between said last-mentioned portmeans unless the fluid pressure at said pressure port means exceeds thepressure at said drain port means by a predetermined amount.

1. In a levelling apparatus for an automotive vehicle having an engine,a hydraulic suspension element interposed between the sprung and theunsprung masses of the vehicle, resilient means biasing said element ina direction to move said masses vertically away from each other, a feedconduit, a return conduit, and control valve means interposed betweensaid element and said feed and return conduits for connecting saidelement to said feed and return conduits respectively in response tochanges in the vertical spacing of said masses, the improvement whichcomprises: a. a shut-off valve arrangement including
 1. a plurality ofport means for connecting the valve arrangement to respective conduitsfor flow of fluid between the valve arrangement and said conduits, saidport means including pressure port means, feed port means, return portmeans, and drain port means,
 2. first valve means operatively interposedbetween said pressure port means and said drain port means andresponsive to a flow of fluid from said pressure port means toward saiddrain port means for moving toward a closing position in which saidfirst valve means seals said pressure port means from said drain portmeans, and thereafter responsive to higher pressure at said pressureport means than at said drain port means for maintaining the closingposition thereof,
 3. second valve means operatively interposed betweensaid return port means and said drain port means,
 4. yieldably resilientmeans biasing said second valve means toward a closing position in whichsaid second valve means seals said return port means from said drainport means,
 5. motion transmitting means interposed between said firstand second valve means for moving each of said valve means away from theclosing position thereof in response to movement of the other valvemeans into the respective closing position,
 6. third valve meansinterposed between said pressure port means and said feed port means;and
 7. yieldably resilient means biasing said third valve means toward aclosing posItion in which said third valve means seals said feed portmeans from said pressure port means, but adapted to yield when the fluidpressure at said pressure port means exceeds the fluid pressure at saidfeed port means by a predetermined amount; b. a pressure pump having anintake and a pressure discharge; c. a container adapted to holdhydraulic fluid; d. a suction conduit connecting said intake to saidcontainer; e. a pressure conduit connecting said pressure discharge tosaid pressure port means; f. a drain conduit connecting said drain portmeans to said container,
 1. said engine being drivingly connected tosaid pump,
 2. said feed conduit being connected to said feed port means,and
 3. said return conduit being connected to said return port means;and g. a body of hydraulic fluid substantially filling said element,said feed conduit, said return conduit, and said control valve means. 2.first valve means operatively interposed between said pressure portmeans and said drain port means and responsive to a flow of fluid fromsaid pressure port means toward said drain port means for moving towarda closing position in which said first valve means seals said pressureport means from said drain port means, and thereafter responsive tohigher pressure at said pressure port means than at said drain portmeans for maintaining the closing position thereof,
 2. In an apparatusas set forth in claim 1, wherein said element has two members movablerelative to each other and jointly bounding a cavity substantiallyfilled with fluid of said body, the improvement further comprising achamber bounded by one of said members and located for collecting fluidleaking from said cavity, a leakage conduit communicating with saidchamber, and a check valve operatively interposed between said leakageconduit and said return port means for connecting the leakage conduit tosaid return port means only when the fluid pressure in the return portmeans is not greater than in said leakage conduit.
 2. said feed conduitbeing connected to said feed port means, and
 3. said return conduitbeing connected to said return port means; and g. a body of hydraulicfluid substantially filling said element, said feed conduit, said returnconduit, and said control valve means.
 3. In an apparatus as set forthin claim 1, pressure relief valve means interposed between said pressureport means and said drain port means for directly connecting saidlast-mentioned port means, and yieldably resilient means holding saidrelief valve means in a closed position and thereby preventing a directconnection between said last-mentioned port means unless the fluidpressure at said pressure port means exceeds the pressure at said drainport means by a predetermined amount.
 3. second valve means operativelyinterposed between said return port means and said drain port means, 4.yieldably resilient means biasing said second valve means toward aclosing position in which said second valve means seals said return portmeans from said drain port means,
 5. motion transmitting meansinterposed between said first and second valve means for moving each ofsaid valve means away from the closing position thereof in response tomovement of the other valve means into the respective closing position,6. third valve means interposed between said pressure port means andsaid feed port means; and
 7. yieldably resilient means biasing saidthird valve means toward a closing posItion in which said third valvemeans seals said feed port means from said pressure port means, butadapted to yield when the fluid pressure at said pressure port meansexceeds the fluid pressure at said feed port means by a predeterminedamount; b. a pressure pump having an intake and a pressure discharge; c.a container adapted to hold hydraulic fluid; d. a suction conduitconnecting said intake to said container; e. a pressure conduitconnecting said pressure discharge to said pressure port means; f. adrain conduit connecting said drain port means to said container,